1. Field of the invention
The present invention concerns a method of monitoring the emission of nitrogen oxides in the exhaust gas of an internal combustion engine.
2. Description of the prior art
Internal combustion engines are a major source of pollutant gases in the atmosphere. They expel combustion residues including nitrogen oxides NO and NO.sub.2 Nitrogen oxides, generically denoted NO.sub.x, are exhausted into the atmosphere in varying amounts according to the engine operating conditions.
The toxicity of nitrogen oxides exhausted by heat engines is fully determined and this has lead various countries to draw up regulations governing the emission of pollutant gases.
The nitrogen oxides comprise nitrogen monoxide NO which is mainly formed in the engine at the combustion temperatures and nitrogen dioxide NO.sub.2 which is obtained when the nitrogen monoxide NO comes into contact with oxygen, in particular the oxygen in the atmosphere.
The concentrations of nitrogen monoxide and consequently of nitrogen dioxide depend on various parameters including: the temperature of the combustion gases, the time available for the reactions to take place and the richness of the fuel/air mixture.
The emission of nitrogen oxides NO.sub.x can therefore be reduced by modifying the parameters which favor their formation. However, it is known that the emission of NO.sub.x is generally maximal when the engine adjustments are optimized from the point of view of efficiency.
As internal combustion engines can operate on multiple sites and/or under different conditions, for example when installed on railroad locomotives, problem arises of being able to determine their nitrogen oxide emission quickly on site and to adjust the engine so as to comply with regulations whilst maintaining the highest possible efficiency.
In some cases it may be beneficial to preserve data on the concentrations of NO.sub.x for subsequent use.
A known way to determine the NO.sub.x concentration of exhaust gas is to use a physical/chemical gas analyzer; this is a complex, costly, heavy, fragile and bulky piece of equipment. Also, this equipment does not lend itself well to computer processing of the measurement results.
What is required is to be able to detect the NO.sub.x concentration using a simple, inexpensive and rugged device which can be installed permanently on the engine, able to operate in real time but also able to supply usable data after a relatively long interval (the data should be storable magnetically, for example). The NO.sub.x concentration and its variations should be measurable relative to a reference level. Finally, it should be possible to apply a correction immediately the NO.sub.x concentration exceeds the permitted standards or drops too low, which may mean that the efficiency of the engine is too low.
The present invention proposes to meet these conditions by using an accelerometer as a sensor and by analyzing the signals supplied by the sensor to evaluate the concentration of NO.sub.x in the exhaust gas of an internal combustion engine by comparison with data supplied by an accelerometer fitted to a control engine.